Comparison of earthworm bioaccumulation between readily desorbable and desorption-resistant naphthalene: implications for biouptake routes.

The bioaccumulation of readily desorbable naphthalene and that of desorption-resistant naphthalene in earthworms were compared to examine the effect of desorption resistance on bioavailability of hydrophobic organic contaminants in soil. A series of naphthalene-contaminated soil samples representing different degrees of desorption resistance were prepared using a batch sorption-repetitive desorption approach, and bioaccumulation of these samples was evaluated using earthworms (Eisenia fetida). Soil samples representing high-degree of desorption resistance exhibited considerably lower bioavailability, as indicated by the lower body burden (naphthalene concentration in worm tissue) at a given sorbed-phase naphthalene concentration. Moreover, the body burden of the highly desorption-resistant samples exhibited a significantly different pore-water dependency than that of the readily desorbable samples, indicating that different biouptake mechanisms are likely controlling readily desorbable contaminants and desorption-resistant contaminants. We propose that for readily desorbable contaminants, the primary biouptake route is the pore-water uptake, but for desorption-resistant contaminants enhanced uptake from ingested soil particles can also be important. The surfactant-like substances in worm gut fluids and physical stress such as abrasion are the likely causes for the enhanced release of desorption-resistant contaminants in worm guts. The difference in bioavailability between readily desorbable and desorption-resistant contaminants needs to be taken into account in risk assessment practices.